1. Field of the Invention
The present invention generally relates to apparatus for holding pipe or other tubular members in a vertical position, and, particularly, to apparatus which is useful in oilfield operations for drilling, setting casing, or placing or removing any tubular member from a wellbore. The present invention increases the strength of drill pipe slip assemblies.
2. Description of the Prior Art
In the drilling or workover of oil and gas wells, it is necessary to thread together numerous links of tubular goods, or pipe. These tubular members may, for example, comprise either a drill string which rotates a bit at the bottom thereof, or a pipe conduit such as production tubing or well casing which is placed and cemented in the wellbore to prevent its walls from collapsing. In the drilling operation, at least some of the weight of the pipe string extending into the well bore is supported by a traveling block and tackle arrangement from a derrick which extends upwardly from the floor of the drilling rig.
When it is necessary to add or remove additional pipe to or from the top end of the drill string, the rotary motion of the drill string is stopped and it is suspended at the floor of the drilling rig while an additional pipe section is threadedly connected to the uppermost pipe section in the drill string. Alternatively, it may be unthreaded and removed from the uppermost pipe section in the drill string. In these instances, the drill string is typically suspended by a drill slip assembly comprising a slip bowl assembly which is mounted in the floor of the drilling rig and through which the drill string extends downwardly into the borehole. The slip bowl assembly has a bore through which the pipe at the upper end of the drill string extends. The slip bowl assembly usually includes a tapered bore such that the bowl is smaller in diameter at the bottom than at the top. The drill slip assembly also comprises a plurality of slip segments (typically three), and the inner portion of each slip segment has a plurality of axial rows of dies, which are gripping elements. The slip segments have an outer taper matches the taper of the bowl. When the slip segments are installed in the slip bowl, inner portions of the slip segments form a cylindrical surface with the gripping elements directed toward the tubular member to be contained in the slip bowl assembly. When the pipe is lowered within the interior of the slip bowl assembly, a camming action between the slip segments of the assembly, and their respective dies, forces the slip segments, and their respective dies inwardly into the pipe, thus gripping it and suspending it from the slip bowl assembly. The slip segments, when installed in the slip bowl, form a cylindrical hole in the center that is roughly the same size as the drill pipe. The slip segments, with their gripping dies protruding radially inward, are manually lowered into the annulus between the bore of the bowl and the drill string when it is desired to suspend the drill string. The assembly naturally grips onto the pipe as it is wedged in the annular taper angle formed between the bowl and the slip segments. When drill pipe is so suspended, an additional joint of pipe may be threadably engaged with the uppermost pipe section on the drill string. The slip segment dies are then removed from engaging contact, and rotary motion is imparted to the drill string to continue drilling.
Also during the drilling operation it may be necessary to remove the drill string to change the bit, to add casing to a portion of the well, or for other reasons. While removing the drill string, rotary motion is stopped and the drill string is suspended in the slip bowl assembly. Thereafter, an elevator which is suspended from the traveling block, in the block and tackle arrangement mentioned previously, is used to grip the pipe just above the slip bowl assembly and the slip segment dies of the slip bowl assembly are disengaged. The traveling block is then raised, the slip bowl assembly slips are reset and the stand pipe extending above the drilling rig floor may be unthreaded and removed. Thereafter, the elevator grasps the pipe extending from the slip bowl assembly, the slip bowl assembly slip segments are again released from contact, and the traveling block again raised. This process may be repeated until the drill string is entirely removed from the wellbore.
Within each slip segment, the axial rows of hardened dies are located for contact with the drill pipe surface. Typically each slip segment has three axial rows of six dies for a total of eighteen hardened dies secured within each slip segment. These hardened dies typically include tooth profiles on the pipe interface surface that enhance the gripping capability of the dies on the pipe by actually penetrating the pipe surface slightly. The hardened dies are necessary because the contact stresses with the pipe can be quite high and the dies are subject to considerable wear.
As the oil industry seeks to drill in ever-deeper offshore waters, the length and weight of the longest drill strings in service have increased accordingly as well as the weight of the suspended loads such as casing strings and liners. As a result of the high repeated loads experienced in many of the deep well applications, bothersome cracking has been noted in the slip segments in the critical xe2x80x9cnosexe2x80x9d areas that support the loads from the dies. If these cracks are allowed to grow to the point of complete failure to support the dies, the result could be the loss of the drill string downhole as well as loss of the suspended load. This could result in huge remedial costs, or complete loss of the well.
U.S. patent application Ser. No. 09/596,489 (xe2x80x9cthe ""489 Applicationxe2x80x9d), which is incorporated herein by reference, discloses a drill slip assembly where each slip segment comprises a load ring attached to the slip segments between an upper and a lower set of dies, and this load ring absorbs stresses imparted by the upper set of dies and protects the lower set of dies from carrying these stresses. The ""489 Application further discloses resilient inserts attached to the top of the uppermost dies of the upper set of dies and the uppermost dies of the lower set of dies. These resilient inserts urge the dies downward and prevent gaps from forming between the dies. Such gaps may yield an unbalanced loading condition among the dies. The apparatus described in the ""489 Application achieves a more uniform distribution of the tubular member load carried by each individual slip segment and its respective dies than attainable using prior art drill slips.
The apparatus described in the ""489 Application provides a substantial improvement in drill slip assemblies in that the nose area has considerable protection from cracking due to an accumulation of axial stress on the lower dies. Even with the apparatus as described in the ""489 Application, however, some nose cracking has still been observed due to lateral stresses along the nose area of the drill slip segments. The nose area of prior art slip segments extends past the supporting bowl such that any lateral movement of the tubular member creates a lateral stress concentration in the nose area. These stresses create cracks along the nose area of the drill slip and cause drilling operators to replace the slips prematurely to avoid a failure of the slip entirely and resulting damage to the drill pipe and possibly the well. Therefore, a drill slip apparatus capable of protecting the nose area from cracking due to lateral stresses imparted by the drill pipe would be desirable to the oil well industry.
In addition, the apparatus described in the ""489 Application utilizes a plurality of axial grooves formed in the drill slip segments to hold the hardened dies. The axial grooves are fabricated using a dovetail cutting tool which cuts a wedge-shaped groove, or dovetail groove, running from the top of the slip segment axially downward to a point just above the bottom of the slip segment. The sides of the wedge-shaped grooves match the sides of the wedge-shaped dies. Because of the shape of the tool, the bottom of the axial groove is rounded with an angled profile, and does not complement the flat bottom of the hardened dies described in the ""489 Application. Therefore, to support the lowermost set of dies which engage the bottom of the axial grooves, prior art assemblies used a half-moon insert which was welded to the bottom of the axial groove. The top of the half-moon insert was flat and complements the bottom of the lowermost set of dies. The bottom of the half-moon insert was rounded and complements the bottom of the axial groove. However, weld failures have been observed on the half-moon inserts during loading operations causing the lowermost set of dies to lose structural support. Therefore, a drill slip apparatus capable of adequately supporting the lowermost set of hardened dies without the use of welded inserts would also be desirable to the oil well industry.
Apparatus in accordance with the present invention is an improvement over the apparatus disclosed in the ""489 Application in the following ways. First, the outward tapered surface of the slip segments is in full contact with the tapered bore of the slip bowl assembly. This result is realized by insuring that the smallest diameter of the slip segment assembly is greater than or equal to the smallest diameter of the tapered bore of the slip bowl assembly.
Second, slip segments in accordance with the present invention are fabricated from forged steel. By using forged steel components, the slip segments function with more durability and with greater load bearing capacity than prior art slip segments fabricated from castings.
Third, in accordance with the present invention, each die in the lowermost set of hardened dies is fabricated having a rounded bottom end with a tapered profile. The rounded end and tapered profile match the shape of the bottom of the axial grooves. This provides full support to the bottom of the lowermost set of hardened dies and precludes the need to weld half-moon inserts to the bottom of the axial grooves.